Glass-ceramic products are well known in various applications. Mention may be made, as examples, of:                electric cooktops (which are generally perfectly flat);        gas cooktops, either flat with cut-outs for the provision of the burners, or having three-dimensional shapes (see notably patent application FR-A-2,735,562);        cooking utensils, such as pans, casseroles, dishes . . .        
The reasons leading to the choice of the glass-ceramic material as constituent material are, essentially:                aesthetic criteria. The glass-ceramics can in fact take numerous shapes, under numerous appearances, either opaque, or transparent and of various colours;        good mechanical strength; and        an excellent resistance to thermal shocks. It is in fact possible to obtain glass-ceramic materials the thermal expansion coefficient of which is very low, even zero.        
The classical methods of obtaining these glass-ceramic products schematically comprise the following steps:                the melting of a glass-ceramic precursor glass (or of a precursor filler of such a glass) in a glass-making oven;        the shaping of said molten glass by a known glass-making method;        the optional implementation of finishing operations (such as external or internal cut-outs, finishing of edges . . . ); and        finally, the implementation of an adequate ceramisation heat treatment, so as to develop the microstructure sought after.        
The shape of these glass-ceramic products is today thus limited by the possibilities of implementation of the shaping methods.
The methods which are mainly implemented hitherto (before the ceramisation: see above) are:                rolling, which is implemented with non-machined laminating rollers, which enables plates of flat surfaces to be obtained, or which is implemented with at least one machined laminating roller, which enables plates with protuberance(s) and/or hollow(s) on at least one of the surfaces, to be obtained;        rolling with suction, which enables protuberances on the plates to be generated (see notably patent application FR-A-2,735,562);        pressing, which enables large varieties of shapes to be obtained (of cooking utensil type mentioned above), but which nevertheless remains limited in these fields of application. It is thus impossible to form, by pressing, pieces of large size and of low thickness.        
In this context of making glass-ceramic products, no welding operation has, to the knowledge of the inventors, been introduced hitherto, neither before, nor after the ceramisation heat treatment.
The welding of glass pieces is a well known method, which is used notably in the manufacture of tubular elements for the chemical industry. Elements such as tubes (cylinders or hollow equivalents), rods (full, rectilinear products), are pre-heated to temperatures which are close to the annealing temperature of the glass considered, and a localised heating is then applied to the zones which are intended to be welded, in order to bring them to temperatures which are high enough to soften them. The pieces to be welded are then placed in contact, at their softened zones. In general, an annealing heat treatment is applied to the welded pieces so as to remove constraints caused by the welding operation.
This type of method (welding) is not applicable to glass-ceramic pieces. The application of localised heating does in fact lead inexorably to notable changes in microstructure, such as growth of the size of the crystals, the formation of crystals of different nature to that desired . . . . Changes of appearance and/or of properties (such as the expansion coefficient) results from this, which strongly deteriorate the quality of the product.
This type of method (welding) was not, a priori, applicable to elements in precursor glass(es) of glass-ceramic(s) either. In any case, a real prejudice existed as to its possible mastery.
The person skilled in the art could reasonably fear an uncontrolled ceramisation of the zones heated (i.e., the formation of zones of different colours or of milky appearance, the expansion coefficient of which differs from that of the rest of the elements in question), a fracturing of the assembly during cooling after welding, the appearance of local constraints which are incompatible with interesting mechanical properties due to the non-constancy of the expansion coefficient value within the assembly . . . .
Now, the inventors of the present invention have shown that it is possible to master such a method of manufacture of glass-ceramic products which incorporates, before the ceramisation heat treatment, at least one welding operation.
U.S. Pat. No. 3,661,546 discloses a method of making telescope mirror blanks by subjecting thermally crystallisable glass polyhedrons to a temperature sufficient to bond together adjoining surfaces of the polyhedrons, followed by nucleating and subsequently crystallizing the glass to form a unitary, thermally crystallized mirror blank.
U.S. Pat. No. 4,248,297 discloses a method of producing a matrix of low-expansion ceramic material comprising non-porous, longitudinal parallel gas passageways therethrough. Such matrix can be regenerators of a gas turbine engine. An embodiment of the method disclosed in this patent reference involves bonding glass tubes to form the matrix followed by thermal crystallization thereof.
The methods disclosed in these references both involve the heating of the whole glass components, such as the polyhedrons and the tubes, when bonding them to form the glass precursor article before ceramization. Such overall heating of the components sometimes are not feasible, or at least not economical in producing certain glass-ceramic articles. A process involving only localized heating of the bonding area is high desired.
In such a context, the present inventors have developed an original method of manufacture of glass-ceramic products having at least one welding implemented on glass-ceramic precursor glass, which involves only localized heating of the welding zone.
This original method notably enables novel shapes of glass-ceramic products to be obtained. In any case, the products obtained by said method, of novel shape or not, are novel in that they bear the signature of their original method of manufacture: the presence of at least one weld joint in their glass-ceramic structure.